1. Field of the Invention
The present invention relates to a process control scheme for cooling and heating compressible compounds, and more particularly, to a process control scheme for making a frozen novelty such as ice cream, frozen yogurt, etc. with an extruding apparatus.
2. Description of the Prior Art
It is known to aerate a mix for the preparation of an ice cream through the use of an aerating apparatus that generally includes a rotating element that fits into the barrel of a continuous ice cream freezer. This aerating apparatus is commonly referred to as a dasher. Rotation of the dasher imparts a mechanical energy into the mix in order to achieve aeration and generate a fat network by aggregating some of the fat droplets. This aggregation is necessary for product stability.
For many continuous industrial freezers, there are a variety of dasher types available. These may be differentiated from each other by the volume displaced within the freezer barrel that may be assessed by simply filling the freezer barrel with a liquid, such as water, and measuring defined by liquid displaced with the dasher is fitted therein. A dasher described as a Series 80 indicates that this rotating element occupies 80% of the available internal volume of the freezer barrel so that only 20% of the space is available to be occupied by the mix to be aerated. By contrast, a Series 15 dasher, also known in the art, demonstrates a displacement volume of only 15% of the internal barrel volume, the remaining 85% being available to be occupied by a mix to be aerated.
In conventional ice cream processing it is generally accepted that higher displacement dashers such as the Series 80 give rise to high quality ice cream being highly churned (Ice Cream 5.sup.th Edition, W. S. Arbuckle et al., page 183) thus showing optimal levels of fat de-stabilization, while at the same time having product dryness, good meltdown resistance and product hardness. These displacement dashers are therefore the standard form of aerating means used in ice cream manufacture.
Traditional frozen aerated products such as ice cream products contain approximately 8 to 12% fat in addition to stabilizers and emulsifiers in order to provide the desired quality product. However, it is now preferable to provide such products that are low fat and that do not include the additives. To date, products provided that are low fat and without added stabilizers and emulsifiers have been inferior in quality in that they are fast melting, have a low percentage of destabilized fat, and are unstable to heat shock, and hence, quickly become very icy. Furthermore, such products have a reduced creaminess perception.
Cold extrusion of aerated compositions is known in the art through the extrusion of a pre-aerated foam through a freezing device. Pre-aeration has conventionally been undertaken through the use of an aerating means in the form of a high displacement dasher. The foam once aerated is then transferred to cold extrusion apparatus. The ice cream extrusion freezer processes ice cream produced in a standard ice cream freezer. The extrusion freezer applies work to the product via shear forces generated by the augers moving the product through the extrusion freezer barrel at very low speeds while removing heat from the product with ammonia as a refrigerant.
The residence time of the ice cream in the extrusion freezer barrel and hence the amount of heat removed from the ice cream may be controlled by varying the speed of the augers. Due to the compressible nature of ice cream, a slower auger speed results in a longer product residence time in the extrusion freezer barrel and therefore greater heat transfer. The rate of heat transfer also depends on the temperature difference between the ice cream and the extrusion freezer ice cream barrel wall. The temperature of the extrusion freezer ice cream barrel wall may be controlled by regulating the temperature of the ammonia refrigerant in the concentric ammonia barrel surrounding the inner extrusion ice cream barrel. A colder ammonia temperature results in more heat being removed from the ice cream moving through the barrel.
Until the present invention, it has not been possible to make frozen novelties such as ice cream of adequate quality with an extruder due to the thickening (or lack thereof) of the ice cream while in the extrusion freezer and the varying pressures that result by altering the temperature. Additionally, the recipe for the frozen novelty affects operation, and ultimately quality of the frozen novelty, of the extrusion freezer.
Several factors are considered in the design of an effective automatic control system for the extrusion freezer including optimizing the work/energy applied to the product, maximizing the heat removed from the product, and leaving the product overrun (volume ratio of mix and air in the ice cream) generated by the standard ice cream freezer unaltered without exceeding system pressures due to excessive ice cream viscosities.